No matter what humans do, it seems that bacteria have an answer. They resist our antibiotics, thrive in our placentas and even eat our oil spills. This evolving bacterial menagerie can cause us many problems, but it may also provide some solutions. Now, a bacterium that eats our drug waste could help stave off our addictions.
Lei Fang and Colleagues at the University of Kentucky in Lexington have carefully altered the structure of an enzyme the bacteria produce: cocaine esterase. The new protein functions at human body temperature for more than 100 days and successfully protects mice against cocaine overdoses. The results suggest that the enzyme could help treat cocaine addicts, both during overdose and to prevent relapse.
Cocaine esterase is a product of the bacterium Stenotrophomonas maltophilia (formerly Pseudomonas maltophilia). Originally isolated from industrial waste at a cocaine-processing pharmaceutical plant, the little microbes use cocaine as food, breaking it down with the help of cocaine esterase into carbon and nitrogen. Cocaine esterase is so efficient that S. maltophilia can persist on cocaine alone if necessary.
Proteins like cocaine esterase are one of three strategies being studied to treat addiction. Replacement drugs, such as methadone for opiate addiction, are much longer lasting than heroin and help prevent withdrawal and craving. Then there are drugs such as the opiate antagonist naloxone that compete for the drug’s receptors to stop the drug of choice from binding. Finally, there are drug candidates like cocaine esterase or antibodies against cocaine that attack the drug itself, preventing it from binding or functioning normally.
While there are drugs on the market for opiate addiction, research for cocaine addiction has been far less successful. “I used to give lectures on drug abuse to medical students,” says James Woods, a behavioral pharmacologist at the University of Michigan, Ann Arbor. “And we never had anything to say about cocaine.”
Cocaine esterase seems promising. But the enzyme has a problem. It functions only when pairs of the protein come together. These pairs, or dimers, are sensitive to temperature. Cocaine esterase is only stable and functional for around 12 minutes at human body temperature — not long enough to be particularly useful. It would be hard to store and transport, and difficult to use.
Fang and colleagues set about trying to make cocaine esterase a more stable dimer at body temperature. They carefully switched out the building blocks of the protein, amino acids, at the site where two enzymes become a matched pair. The scientists have already been able to create a version of cocaine esterase that can withstand human body temperature for six hours. And it’s already in a phase II clinical trial to treat cocaine overdose.
While six hours might be enough to prevent a drug death, it’s not long enough to give to an addict to prevent them getting high during a relapse. So the scientists kept tweaking the proteins, and in a paper published June 11 in ACS Chemical Biology, the researchers show that they can make cocaine esterase far stronger than before. When they replaced the amino acids at the pairing site with cysteines, the bonds between the two amino acids become much stronger, allowing the pair of proteins to stay together. The net result is a protein pair that persists at human body temperature in a dish for more than 100 days.
The more stable protein pair came with a side effect: It was 150 percent more effective than the original six-hour version at breaking down cocaine into benzoic acid and ecgonine methyl ester, chemicals that can’t get anyone high. The change to the dimer also changed the active site that breaks down cocaine, explains Chang-Guo Zhan, a medicinal chemist at the University of Kentucky and an author on the study.
Experiments in mice showed that the enzyme could protect them against massive cocaine doses (180 times the normal amount taken by humans) for around three days. Hopefully, it would be effective against normal doses for a far longer period of time. “This is the longest in vivo protection from a lethal dose of cocaine ever demonstrated,” says Laurent Karila, an addiction psychiatrist at Paul Brousse Hospital in Paris. “Other studies are needed but it could be promising in cocaine-addicted people.”
Cocaine esterase could also be used to keep addict from getting high. “You can depend upon an addict, regardless of his intentions, to try the drug again because of the tendency to relapse,” Woods notes, “so you want to have something that minimizes that problem.” With an enzyme present in the body, Zhan explains, “any time a cocaine addict takes cocaine again, it will be metabolized rapidly before it can produce any stimulant effects.”
The 100-day half-life of the modified enzyme is crucial. “The longer the half-life, the less frequently one needs to have the enzyme injected,” Zhan explains. But Woods cautions that drug treatment alone is probably not enough, and that treatment should be paired with changes in the addict’s environment and behavior. He notes that “you have to stack the deck in favor of abstinence.”
But Woods remains upbeat. “It’s one of them most exciting things that’s come along in my career,” he says. It’s particularly impressive that such an effective drug weapon evolved in a relatively ordinary bacterium. “It’s amazing what Mother Nature has provided us that way,” Woods notes: “Absolutely amazing.”